Problem: In geometry class, the girl to boy ratio is $9$ to $3$. If there are $12$ boys, how many girls are there?
Solution: A ratio of $9$ girls to $3$ boys means that there is a group of $9$ girls for every group of $3$ boys. If there are $12$ boys, then there are $4$ groups of $3$ boys each. Because we know the ratio is $9:3$ , there must be $4$ groups of $9$ girls each. There is a total of $36$ girls in geometry class.